A British rugby union player's opening try is most unexpected (9)

'most unexpected' is the definition.
Both the answer and definition are superlative adjectives. Perhaps they are linked in a way I don't understand?

'a british rugby union player's opening try' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'rugby union' becomes 'RU' (abbreviation).
'opening' suggests taking the first letters.
'try' becomes 'test' (test is a kind of try).
The first letter of 'players' is 'p'.
'a'+'b'+'ru'+'p'+'test'='ABRUPTEST'

'is' acts as a link.

Can you help me to learn more?